1. Field of the Invention
An object of the present is a method to depict moving parts in a body using a nuclear magnetic resonance (NMR) imaging method. The invention can be applied especially in medicine, where the bodies examined are human bodies and where the moving parts may be blood cells flowing in the veins or arteries, or moving organs such as heart muscle. In particular, this invention can be used for angiographic analysis. In this way, it becomes possible to plot the vascular system of a body.
2. Description of the Prior Art
There are known prior art methods, especially as described in the European patent No. 84 307746.2, filed on 19th November 1984, for depicting the images of the moving parts of a body by performing two successive experiments. In a first series of experiments, the image of a section of the body is acquired in a standard way. It is known that the image of the moving parts of the body is not revealed because the contributions provided by these moving parts to the NMR signal are uniformly distributed in phase: these contributions cancel one another out. It has been shown that the cancellation of these contributions is related to the very shape of the magnetic encoding undergone by the parts to be imaged during an imaging sequence. By using so called compensating bipolar pulses to compensate for the unwanted effect of these encodings on the moving parts, it is possible to obtain, in a second series of experiments, an image that reveals both the fixed and the moving parts of the body. By subtracting the standard image from the image obtained with compensation, it is then possible to obtain the image of the moving parts only.
While this method is an effective one, it has one drawback: it is lengthy. For the acquisition of the image of the moving particles calls for the acquisition of two images: the compensated image and the speed encoded image. Now, in NMR, the acquisition of an image is a lengthy process. For the imaging methods used take into account the fact that the response restituted by the body, at each excitation that it undergoes, is a response in volume: at the end, all the particles emit a de-excitation signal. It becomes necessary then to repeat the experiments in series of excitation sequences during which the encoding varies from one sequence to another so as to enable the decoding of the image. Since the return to equilibrium of the magnetic moments of the particles excited during excitation is relatively slow, one sequence can begin after another sequence only after a period of time during which it may be assumed that this equilibrium is re-established. The precision of the images is directly related to the number of sequences in a series of sequences. Usually, the duration needed to acquire an image is about 8 minutes. Thus, an angiographic operation lasts about 16 minutes.
An object of the present invention is to remove this drawback by proposing an image of moving parts which can be acquired in half this time. In the invention, it has been seen to it that, instead of compensating for and encoding the effect of the moving particles during the sequence, the radiofrequency excitation of these particles is sensitized before the acquisition/reconstruction process so that only moving particles contribute to a measurable NMR signal. The fixed parts have their magnetic moments reset and realigned with the orienting, homogeneous, constant, magnetic field of the machine. Other advantages result from this. In the phase encoding sequences within the sequence, the contrast in flow is obtained only after the entire acquisition/reconstruction process. Thus any errors, especially phase errors in the NMR signal, due to eddy currents in the coils used to apply the encodings, are included in it. In the invention, the errors in the processing to reconstruct the image are reduced because only one image is computed. Finally, the image can be sharper.